FNV1 Algorithm giving Arithmetic Operation Overflow

[Carl Kruger]

I am trying to make a small programm that will convert a given input string to a FNV1 32bit hash code. I found a function algorithm to do the actual convertion.

public static uint ComputeHash(IEnumerable<byte> bytes)
{
   uint hash = 2166136261;
   foreach(byte b in bytes)
   {
      hash *= 16777619;
      hash ^= b;
   }
   return hash;
} 
 

From what I have been able to read about and comparing to similar functions, this appears to be correct. But no matter what I put into the function I get a "arithmetic operation resulted in an overflow" OverflowException evertime, I am putting in a byte array equivelent of the input string. How do I fix this issue ? Am I just misunderstanding something or is the function faulty(more likely the former) ?

[GaryTexmo]

Can you post your calling code too? I just duplicated this and tried it with a few strings and am not getting the error.

class Program
{
    static void Main(string[] args)
    {
        Console.WriteLine(GetHash("abcd".Select(x => (byte)x).ToArray()));
    }
 
    static uint GetHash(IEnumerable<byte> bytes)
    {
        uint hash = 2166136261;
 
        foreach (byte b in bytes)
        {
            hash *= 16777619;
            hash ^= b;
        }
 
        return hash;
    }
}

[Carl Kruger]

For now I have window form with 2 textboxes

• tbHashIn
• tbHashOut

And a button. In the buttonclick even is the following

string inputString = this.tbHashIn.text;
byte[] inputBytes = new byte[inputString.Length];
For (int i=0;i<inputBytes.Length;i++)
{
   inputBytes[i] = (byte)inputString[i];
}
uint outint = ComputeHash(inputBytes);
this.tbHashOut.Text=outint.ToString(); 

[Carl Kruger]

sorry typo line 1 is

string inputString =this.tbHashin.Text;

[GaryTexmo]

So tbHashin is a textbox on your form and you're just passing the text through? What kind of things are you testing with?

Can you try copy/pasting the code I wrote (into a new Console Application) and seeing if that works? I did try some longer strings, but nothing over 200 characters.

Looking at the code, that you get an overflow exception isn't that surprising because you keep multiplying by 16777619, but I looked at it and the bits just overflow and reset to a lower number. There is a way to force overflow checking though... and it's also a project setting. The first thing to check is your project's properties... click the Build tab, scroll down and click the Advanced button. Uncheck Check for arithmetic overflow/underflow if it is checked.

Alternatively, if you want to leave that setting enabled, you can specifically check or not check commands by putting them in checked or unchecked blocks. So for your function there, put the whole thing in something like this...

unchecked
{
  // Your code here
}

[Carl Kruger]

the text box entries are only about 20-30 characters on average with a max of about 40. I wrapped the code in the get hash function in a unchecked block and the program now seems to be working fine without any crashing (I forgot about those). Now I just need to find a way to confirm the hash ouput is correct (I'll check the net).
Thanks again for all your help on what should have been a simple small program (I've made larger more complex ones). It's been educational.
Thanks
CDK